Mobile Noise

[Detroit47]

I have a Problem with alternator noise coming into my mobile radio via the power line. It isn’t coming in thru the antenna so I am thinking that it is alternator ripple. I have made up a large bifilar choke along the lines of a filament choke but that didn’t get it all. I am thinking about an iron core choke but I don’t know a good source. I need to be able to pull about 25 amp dc thru it. Has anybody had success using a transformer for a choke? I also had the thought of winding a choke along the lines of a speaker cross over choke. I am open to any and all ideas this noise is annoying. I have the same problem on my 2 meter 440 rig as well as my hf rig.
Thanks John N8QPC

[k9jri]

John, the first thing to suspect it the battery itself as it is the primary filter in the system.  If it is older or weakened it will do a poor filtering job.

The second item is the ground return from the battery to the radio equipment.  If you do not have a negative wire from the battery to the radios one should be added.  Using the automobile/truck frame as the ground return will also introduce additional noise into the system.

As regards your filter it is a good idea to place a large capacitor between the Plus and Minus leads on the radio side of the filter to act as a low AC impedance across the output of the filter choke.  Without this capacitor the choke will work quite poorly.  Same as a filter cap in a power supply

73 - Mike - K9JRI

[KA0HCP]

Alan, K0BG has a fantastic website with everything there is to know about mobile setups.

www.k0bg.com

[WU2D]

My old Maxima had fuel pump noise that dominated on 75M but for Alternator whine, I made a common mode choke out of an old transformer core and wound as much #12 bifilar on it as I could fit and then went into a 5,000 uF 50 VDC Cap. Don't forget to fuse both hot and ground inline. Good luck!

[ke7trp]

I had an HD Ford pickup with a mobile. It was a nightmare.  full scale noise.  I used snap on RF chokes purchased at a ham fest.  I wrapped the power cord to the radio through two of them. This lowered the noise.  I choked the alt lead, The pos and neg bat lead, and other cables under the hood.  The last of it was the fuel pump. I crawled under the truck and clipped three on the fuel pump line.  The noise was S2.  I think I used about 15 of those Clip on chokes.  Lucky for me, I got a bag of 20 of them for 15 bucks. 

Later,  I got a 5 farad 20 volt capacitor from a friend.  This cap is very small for 5 farad.  About 3.5 inches square. It was being marketed for an increase in fuel mileage for a car.  Odd..  In short, It did not work and I ended up with it.  That cap is truly 5 farad.  we tested it with a known load.  That cap, Across the battery removed every possible trace of noise.

C

[KB3WVB]

All the above suggestions are a great place to start.  I operate primarily mobile so I've had to deal with similar problems.  K0BG's website provides a WEALTH of information on mobile operation and dealing with mobile problems.

Here is what I would do:

1)  Make sure you are connecting directly to the battery.  Both the positive and negative (ground) terminals.

2)  Permanently mount your antenna.  Mag-mounts don't provide a proper RF ground for the antenna. (I don't know if you have one or not but they do seem popular).

3)  Choke your coax at the point where it leaves the vehicle.  This may or may not help with you on the receive end but if you are transmitting your alternator noise, this will get rid of it.  An easy way to do this is to get a large Jiffy peanut butter container, then tightly wrap your coax around it and mount it inside the vehicle as close to the point where the coax leaves the vehicle interior.

4)  RF bond your vehicle to the chassis.  Details can be found on k0bg.  This takes a lot of time but it is a BIG help.


Finally, if any or all the fixes above don't work for you (or are impractical), check to see if your radio has a noise blanker function.  This is kind of a band-aid fix but it will get rid of a lot of excess impulse noise (like an anternator, spark plugs, etc).  Most new radios have this function and it does work well.  The tradeoff is that you will lost a bit of your receive capability.

[WB6NVH]

In the mobile radio industry it is considered bad practice to bring the negative lead from the radio directly to the battery.  This, because if there is a problem in the main ground from the battery to the block, the starter can try to draw current through the radio.  Thus better to attach the ground near the battery but not directly to it.  Or perhaps fuse the negative side as well.

[KB3WVB]

True, you are right about the fusing on the negative side.  Dealing with mobile installs on new vehicles is an absolute pain.  Unfortunatelly, I have found that attaching to a ground point near the battery does very little to avoid the inevitable ground loop.  The amount of RF in new vehicles is absolutely insane.  Your radio is competing with upwards of 50 processors and their associated circutry/wiring.  Avoiding the much more likely possibility of a ground loop by making a direct battery connection becomes much more preferable.  Most newer radios are also trending in this direction.  Go pick up a Yaesu manual and it recommends directly connecting to the battery.  Just like you said though, Yaesu incorporates a fuse on each wire to avoid any unfortunate issues.

[n1ps]

Alternator noise will most likely not get filtered out with a choke but by a cap.   The cap needs to be BIG, at least 10K uF and it needs to be located as close to the noise source (alternator) as possible with short leads.  BTW....when you hook it up, watch your polarity.  If you reverse it that cap becomes a bomb     Guess why I know this?   

The mobile industry tends to connect the neg lead to the body which is OK as we are talking FM and on VHF/UHF.  On HF, ground loops and noise become the main concern and we should run the neg current carrying wire to the battery.
~ps